Process of making a laminated spinneret

ABSTRACT

A corrosion-resistant spinneret, suitable for spinning filaments of poly(p-phenylene terephthalamide) from a viscous solution of the polymer in 98-100 percent sulfuric acid, is prepared from a plate of tantalum alloy or stainless steel which is clad on one face with a layer of pure tantalum. A spinneret blank is formed, counterbores are drilled through the plate and partially into the tantalum layer, the blank is annealed, spinneret capillaries are formed from the counterbores through the tantalum layer to the exit face of the spinneret, the face is polished to remove protrusions, and the face is hardened by heat-treatment in nitrogen. Uniform capillaries having a diameter of about 0.002 inch (0.05 mm) and a length greater than about 0.005 inch (0.13 mm) can be formed in spinnerets without breaking tools used in their production.

BACKGROUND OF THE INVENTION

This invention relates to production of corrosion-resistant spinneretsand, in particular, to spinnerets suitable for spinning filaments ofpoly(p-phenylene terephthalamide) from a viscous solution of the polymerin 98 to 100 percent sulfuric acid.

Blades U.S. Pat. No. 3,767,756 discloses production of filaments havingremarkably high tenacities from poly(p-phenylene terephthalamide) andrelated wholly aromatic polyamides (aramids) by spinning a viscoussolution of the polymer in 98 to 100 percent sulfuric acid throughspinneret capillaries 2 to 4 mils (0.05 to 0.10 mm) in diameter.Chlorosulfuric and fluorosulfuric acid are also disclosed as suitablesolvents. When such highly corrosive solutions are extruded throughfilament-forming capillaries in spinnerets composed of customarymaterials, corrosion soon makes the spinnerets useless. An initialeffect is that the originally sharp edges, defining the outletperipheries of the capillaries, become dulled and rounded. This causeserratic non-uniformities of denier in the spun filaments, and threadlinebreaks occur at increasing frequency.

Tantalum has excellent corrosion resistance and has been proposed forspinnerets used in spinning rayon by the viscose process. Austin U.S.Pat. No. 1,791,785 teaches that tantalum is too hard for properlydrilling spinneret holes unless it has been softened or annealed, butthen the holes are likely to be deformed by subsequent use of thespinneret. Austin proposes coating the spinneret with an electrolyticfilm. British Pat. No. 702,936 mentions previous attempts tosurface-harden completed tantalum spinnerets by heating them in air,oxygen, nitrogen or carbon monoxide, but states that such treatmentsimpair the quality of the spinning passages to such an extent as tocause them to get rapidly blocked during spinning. Another difficultywith annealed tantalum is that it is a yield strength of less than30,000 pounds per square inch and an elongation of at least 20 percent,i.e., is quite ductile. Particularly for the present purpose, anexcessively thick tantalum spinneret would be required to avoid bulgingat the high spinning pressures used. Optimum fiber properties are alsodifficult to obtain because of the length of spinning passages through athick spinneret.

Hull U.S. Pat. No. 2,965,924 proposes forming spinneret holes in thinsheet metal, which may be a noble metal, drilling holes in acorresponding pattern in a spinneret blank, plating the blank withcopper or silver, assembling the punched sheet metal with the platedblank, and brazing the assembly together. There are several difficultieswith this procedure. (1) Precise alignment of holes produced in theseparate parts is too difficult. (2) Excess of the copper or silverbrazing metal can partially or completely block spinneret capillaries.(3) Deformation of the assembly occurs during brazing. (4)Discontinuities in the brazing metal allow leakage to occur between thesheet metal and the blank. Ogden et al. U.S. Pat. No. 3,279,284 proposesavoiding the use of brazing metal by instantaneous welding, e.g.,explosively bonding the parts together by detonating a sheet ofexplosive material. However, adequate bonding in this manner inevitablycauses some deformation of the relatively soft face layer containing thespinneret capillaries; repolishing the face to a flat surface thenintroduces non-uniformities in the lengths of the spinneret capillarieswhich cause objectionable denier variations in the filaments produced.

SUMMARY OF THE INVENTION

The present invention provides a laminated spinneret and process ofmaking it which avoid the indicated difficulties with previouscorrosion-resistant spinnerets. Spinnerets can be prepared that aresuitable for spinning filaments of poly(p-phenylene terephthalamide)from viscous solutions of the polymer in 98 to 100 percent sulfuricacid, have excellent resistance to corrosion, and have uniform spinneretcapillaries which have improved resistance to dulling and rounding ofthe outlet peripheries.

Preferred embodiments of the invention comprise the process steps of

a. machining a laminate, consisting of a plate of tantalum alloy orstainless steel clad on one face with a layer of commercially puretantalum, to form a spinneret blank wherein the area of the blank whichis to contain spinneret orifices has parallel flat faces, is of athickness sufficient to prevent bulging of the spinneret face atspinning pressures, and has an exit face layer of tantalum about 0.025to 0.04 inch (0.6-1.0 mm) thick,

b. drilling a counterbore through the plate and partially into thetantalum layer at the location of each spinneret orifice,

c. forming a conically tapering section at the bottom of eachcounterbore,

d. wrapping the spinneret blank in tantalum metal foil and heat-treatingthe blank in vacuum at about 1,800° to 2,000° F (980°-1,100° C) toprovide a soft-annealed tantalum layer,

e. forming an axially aligned spinneret capillary, of about 3 to 12square mils (0.002-0.008 mm²) in cross-sectional area, from the tip ofeach conical taper through the tantalum layer to the exit face of thespinneret blank,

f. polishing the exit face of the spinneret to remove protrusions formedby the punching operations, and

g. heat-treating the spinneret in nitrogen at about 1,600° to 1,700° F(870°-930° C) to harden the face.

Spinneret capillaries of any desired cross-section can be formed in step(e). Capillaries having a diameter of about 2 to 4 mils (0.05-0.10 mm)and a length at least about 2.5 times the diameter are generallypreferred.

The tantalum-clad plate used to form the spinneret blank in step (a) isprepared by known methods of welding one metal to another by a thintransition layer composed of a mixture of the two metals. The weld ispreferably achieved by explosive bonding as disclosed in Cowen et al.U.S. Pat. No. 3,137,937. The laminate preferably consists of a layer ofcommercially pure tantalum explosively bonded onto a plate of tantalumalloyed with 2 to 3 percent of tungsten based on the weight of tantalum.

The invention also comprehends spinnerets wherein the exit face layer isan alloy of gold or platinum, although such noble metal alloys are muchmore expensive than tantalum. The corrosion-resistant spinneret maycomprise a backer plate of tantalum alloy or stainless steel, anexit-face layer of a material selected from commercially pure tantalum,a gold-platinum alloy, a gold-platinum-rhodium alloy, agold-platinum-rhenium alloy, and a gold-rhodium alloy, the exit facebeing about 0.025 o 0.04 inch (0.06-1.0 mm) thick and being bonded tothe backer plate by a relatively thin transition layer composed of amixture of the metals used in the backer plate and exit-face layer, acounterbore extending through the backer plate and partially into theexit-face layer, a conically tapering section at the bottom of thecounterbore, a spinneret capillary extending through the exit-face layerfrom the conically tapering section, the cross-sectional area of thecapillary being about 3 to 12 square mils (0.002-0.008 mm²) andsubstantially less than that of the counterbore, and the length of thecapillary being greater than about 5 mils (0.13 mm).

Preferably the tantalum alloy or stainless steel of the backer plate ischaracterized by having an annealed yield strength of at least 80,000pounds per square inch (5,625 kg/cm²), an elongation of at least 10percent, and a corrosion rate of less than 1 mil (0.025 mm) per year insulfuric acid.

Preferably the commercially pure tantalum or gold alloy of the facelayer is characterized by having an annealed yield strength of less than30,000 pounds per square inch (2,110 kg/cm²), an elongation of at least20 percent, and a corrosion rate of less than 1 mil (0.025 mm) per yearin sulfuric acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of a round, cup-shapedspinneret embodiment of this invention.

FIG. 2 is a greatly enlarged cross-sectional view of a portion of FIG. 1which includes a single spinning passage.

DETAILED DESCRIPTION

Spinneret 10 is composed of a backer plate 12 and an exit-face layer 14.The backer plate is made of tantalum alloy or stainlesss steel. Theexit-face layer is preferably made of commercially pure tantalum but maybe a noble metal alloy. As shown in FIG. 2, the face layer has beenexplosively bonded to the backer plate, producing a wave-like interface22 where the metals have intermixed. Spinning passages 16 extend fromentrance face 18 to exit face 20 of the spinneret. The two faces areflat and parallel. There will usually be many more spinning passagesthan those shown. A spinning passage includes a cylindrical counterbore24 drilled through the backer plate and partially into the face layer asindicated by the dashed line 30, a conical taper 26 formed by punchinginto the bottom of the counterbore to a depth indicated by dashed line32, and a spinneret capillary 28 formed through the remaining thicknessof the face layer. The capillary shown has a length of about three timesits diameter. The face layer 14 must be thick enough to accommodate allof conical taper 26 and capillary 28; it is typically of the order of0.025 to 0.04 inch (0.6- 1.0 mm) in thickness. The thickness of thebacking plate in the area having spinning passages is ordinarilysomewhat greater than that of the face layer, but is preferably nothicker than is sufficient to prevent bulging of the spinneret face atspinning pressures which will be experienced in use.

Spinnerets of this invention are particularly useful for spinningpolyamide filaments of unusually high tenacity as disclosed in BladesU.S. Pat. No. 3,767,756. The process requires the use of highlycorrosive solvents such as 98% sulfuric acid, chlorosulfuric acid,fluorosulfuric acid, and their mixtures. Moreover, the rheology of thepolymer solutions is such that preferred tensile properties are obtainedwhen using spinning capillaries of small diameter (D) and highlength-to-diameter ratios (L/D). Typical capillary diameters are 2 to 4mils (0.05 to 0.10 mm) at L/D ratios of at least about 2.5. Preferablythe diameter of the counterbore is from 6 to 12 or more times thediameter of the spinning capillary, and the length of the counterbore(including the conical taper joining the counterbore to the spinningcapillary) is about 2 to 8 times the diameter of the counterbore. Thesesizes, coupled with the high flow viscosities, require high extrusionpressures.

The laminated spinneret for this purpose may be of the plate, disc, orflat-bottomed cup type having spinning capillaries of round or non-roundcross-section which intersect the exit face perpendicularly. The backerplate is composed of metal having an annealed yield strength at a 0.2%offset of at least 80,000 psi (5,625 kg/cm²), an elongation at one inch(2.54 cm) gage length of not less than 10%, and a corrosion rate notgreater than 1.0 mil/year (0.025 mm/yr). The exit-face layer is composedof a metal having an annealed yield strength at 0.2% offset of less than30,000 psi (2,110 kg/cm²), an elongation at one inch (2.54 cm) gagelength of not less than 20%, and a corrosion rate not greater than 1.0mil/year (0.025 mm/year). Tensile strength in pounds per square inch(psi) and percent elongation are determined according to ASTM StandardE8-69 using the Rectangular Tension Test Specimen. The corrosion rate isdetermined by the 168-hour immersion test according to ASTM StandardG31-72, using concentrations of sulfuric acid up to 100% at temperaturesfrom 0° to 100° C.

The preferred material for the backer plate is tantalum alloyed with 2to 3 percent of tungsten. Alloys of this type which also contain a grainrefining agent such as niobium are less desirable because machining ismore difficult. Alloys of tantalum containing up to 15 percent by weightof tungsten may be used but are difficult to machine, and counterboredrilling results in high tool wear. Stainless steels having the requiredyield strength, elongation, and corrosion resistance may also beemployed.

The preferred material for the exit-face layer is commercially puretantalum. After cladding, its yield strength will exceed 30,000 psi.(2,110 kg/cm²) and the formation of high L/D spinning capillaries is notfeasible commercially. When it is annealed while wrapped in tantalumfoil at about 1,800° F (982° C) under vacuum, however, its yieldstrength is reduced to below 30,000 psi (2,110 kg/cm²), and high L/Dspinning capillaries can be formed with care on a commercial basis.Because its yield strength is close to 30,000 psi, the exit-face layeralso contributes to the resistance of the spinneret to deformation underpressure. Moreover, its surface can be superficially hardened so as tobetter resist scratches during use by heating in a nitrogen atmosphereat about 890° C for sufficient time (ordinarily about 1 hour).

Other materials than tantalum can be used for the cladding layer so longas they have the required corrosion resistance and annealed yieldstrengths of less than 30,000 psi (2,110 kg/cm²). Among such suitablematerials, listed in order of increasing hardness, are gold, M-metal(90% gold/10% rhodium by weight), C-metal (69.5% gold/30% platinum/0.5%rhodium by weight), D-metal (59.9% gold/40.0% platinum/0.1% rhenium byweight), and Z-metal (50.0% gold/49.0% platinum/1.0% rhodium by weight).The latter was substantially the same hardness as tantalum. Alsosuitable is a 75% gold/25% platinum alloy. All of these metals are,however, much more expensive than tantalum. All but Z-metal are muchmore easily damaged in use than tantalum. Softer materials areadvantageous, however, when capillaries of quite high L/D ratio (e.g.,greater than 3.5) are to be formed.

Explosive bonding is preferably achieved as taught by Cowan et al. inU.S. Pat. No. 3,137,937. The two ductile metal layers to be bonded arearranged to be parallel and separated by at least 1 mil (0.025 mm), anexplosive having a detonating velocity greater than 1,200 m/sec but lessthan 120% of sonic velocity is placed uniformly over one outsidesurface, and the detonation is propagated parallel to the layersstarting at one edge of the explosive-bearing layer. A continuousmetallurgical bond results at the interface characterized by a wave-likeintermixing of the two metals.

Some plastic deformation of the laminae is an inevitable result ofexplosive bonding. Therefore, as a minimum, machining the laminaterequires grinding and polishing of the exit and entrance faces to renderthem flat and parallel. Precise machining of the periphery to form theshape and size of a spinneret blank is also normally required.Frequently, machining involves converting the laminate to a cup-shapedspinneret blank of the type illustrated in FIG. 1.

In the area of the spinning blank selected for formation of spinningpassages, a counterbore for each spinning passage is drilled in knownfashion. Initial tapers with relatively large included angles may alsobe drilled at this stage. Each of the resulting holes would extend auniform distance from the entrance face of the spinneret blank to adepth at which the generally horizontal bottom surfaces of the hole arecompletely within the cladding layer. The final taper (smallest includedangle) is then formed by punching at the bottom of each hole. If thedrilled hole does not extend into the cladding layer, punching of thefinal taper coins metal from the backer plate along the walls of thetaper, thus hardening the walls and causing breakage of punchessubsequently used to form spinning capillaries. Should any of theseoperations produce protuberances on the exit face, the exit face shouldbe polished to flatness before proceeding.

Before spinning capillaries are formed in the cladded facing metal, thespinneret blank should be softened by annealing in order to minimizebreakage of formation tools. The material of the cladding layer shouldhave an annealed yield strength of less than 30,000 psi (2,110 kg/cm²)to avoid breaking formation tools, particularly when forming capillarieshaving L/D values of at least about 2.5. Time and temperature conditionsfor annealing the cladding materials are well known. When the preferredtantalum is annealed in the presence of oxygen, hard oxidized anddiscolored surfaces result. To avoid these drawbacks, a spinneret blankwith a tantalum cladding layer is first wrapped tightly in tantalummetal foil and then annealed in a vacuum at about 1,800° F (982° C). Aspinning capillary of any desired cross-sectional shape is formed in theannealed blank through the remaining thickness of cladding layer at thebottom of each hole. The exit face is then polished to removeprotrusions formed.

Formation of spinneret capillaries is accomplished with a formationtool, i.e., an elongated punch-like tool having at its tip a smallextension longer than but of precisely the same cross-section as thedesired capillary. The extension of the formation tool is pushed throughthe thickness of spinneret plate remaining between its exit face and thebottom of a preformed counterbore hole. This operation is akin topunching and is often referred to as such.

The softer the material of the cladding layer, the more readily it isdamaged in handling. Thus, the completed spinneret should be hardened tothe extent possible. For most materials this involves heating to atemperature close to the annealing temperature followed by slow coolingin known fashion. Tantalum is preferably surface hardened by heating ina nitrogen atmosphere at 890° C for about 1 hour.

We claim:
 1. In the production of a corrosion-resistant spinneretsuitable for spinning filaments of poly(p-phenylene terephthalamide)from a viscous solution of the polymer in 98 to 100 percent sulfuricacid, the process steps which comprisea. machining a laminate,consisting of a plate of tantalum alloy or stainless steel clad on oneface with a layer of commercially pure tantalum, to form a spinneretblank wherein the area of the blank which is to contain spinneretorifices has parallel flat faces, is of a thickness sufficient toprevent bulging of the spinneret face at spinning pressures, and has anexit face layer of tantalum about 0.025 to 0.04 inch thick, b. drillinga counterbore through the plate and partially into the tantalum layer atthe location of each spinneret orifice, c. forming a conically taperingsection at the bottom of each counterbore, d. wrapping the spinneretblank in tantalum metal foil and heat-treating the blank in vacuum atabout 1,800° to 2,000° F to provide a soft-annealed tantalum layer, e.forming an axially aligned spinneret capillary, of about 3 to 12 squaremils in cross-sectional area, from the tip of each conical taper throughthe tantalum layer to the exit face of the spinneret blank, f. polishingthe exit face of the spinneret to remove protrusions formed by thecapillary forming operations, and g. heat-treating the spinneret innitrogen at about 1,600° to 1,700° F to harden the face.
 2. A process asdefined in claim 1 wherein spinneret capillaries of a diameter of about2 to 4 mils and a length of at least about 2.5 times the diameter areformed in step (e).
 3. A process as defined in claim 1 wherein saidlaminate consists of a layer of commercially pure tantalum explosivelybonded onto a plate of tantalum alloyed with 2 to 3 percent of tungstenbased on the weight of tantalum.